The present invention relates generally to circuit board construction, and more particularly relates to apparatus and methods for forming an electrical grounding connection between a circuit board and a chassis to which it is secured.
When mounting a circuit board on a metal chassis, such as a printed circuit board mounted on a computer chassis, it is necessary to form an electrical grounding connection between the chassis and the ground plane of the circuit board. One previously used technique for effecting this circuit board/chassis grounding connection is to form a spaced series of metal-plated through mounting holes in the circuit board. During the subsequent wave soldering process, solder wicks up into the plated mounting holes and fills them. Before the holes can be used, the solder filling them must be reamed out. Mounting screws are then extended through the reamed-out holes and threaded into the underlying chassis. While this technique forms an adequate grounding connection between the circuit board and the chassis, it is considered undesirable due to the time required to ream out the solder-filled mounting holes which undesirably increases the overall fabrication cost of the circuit board.
In another conventional grounding technique, the mounting holes are left unlined, and connection to ground is achieved by forming a spaced series of metal-plated grounding holes or "vias" through the circuit board around each of the unlined mounting holes, with each of the vias extending through the circuit board ground plane. Metal plating layers are also formed on opposite side surfaces of the circuit board and electrically couple the opposite ends of the internal metal plating portions of the vias. During the subsequent soldering process, solder wicks into the vias and forms outwardly projecting solder grounding pads on their opposite ends along the top and bottom sides of the circuit board substrate.
The theory of this previously used grounding construction technique is that when the bottom side of the circuit board is placed against the chassis, and mounting screws are extended through the unlined mounting holes and threaded into the chassis, the bottom side solder grounding pads will be brought into forcible contact with the underlying chassis and thereby ground the circuit board thereto. While this method overcomes the added cost factor of the previously described grounding method, it has the inherent disadvantage that the thicknesses of the resulting solder pads on the bottom side of the circuit board cannot be accurately controlled due to the upward wicking of the solder into the grounding vias. This results in the bottom side solder grounding pads having varying thicknesses. Accordingly, this solder pad thickness variation at the circuit board/chassis grounding interface along the bottom side of the circuit board results in fewer grounding contacts being made to the chassis, thereby enabling large ground loops that pose problems from an emissions standpoint.
As can be readily seen from the foregoing, it would be desirable to provide improved apparatus and methods for forming a circuit board/chassis grounding connection that eliminate or at least substantially reduce the problems, limitations and disadvantages associated with conventional grounding techniques such as those generally described above. It is accordingly an object of the present invention to provide such improved apparatus and methods.